The existing power frequency induction devices for liquid heating can be divided into two types, i.e. current heating and eddy-current heating, by their working principle of heating, as is referred to in European Patent EP0383272A2 and Chinese Patent ZL97106984.4.
The working principle described in European Patent EP0383272A2 is: when the primary winding is connected with an industrial frequency power source, a low voltage-high current is induced in the metal pipes as the secondary side so that the metal pipes are heated and the heat is conducted to the liquid. Its structure is: the iron cores are all laminated by silicon steel sheets, which surround the primary winding and the metal pipes as the secondary side one by one from inside out on the core legs of the iron core to form an integral part that goes through the liquid container. A resinous mold is filled out between the primary and secondary sides so that the vacancy that is unfavorable for heat conduction is eliminated and uniform heat generation from the surfaces of the secondary metal pipes is made possible. It is obvious in this structure that the core legs of the iron core are put through the container together with the secondary metal pipes to form a closed magnetic loop with the upper and lower yokes outside the container. Thus, in terms of the relations between power and safety voltage, a lower power will lead to a lower voltage on the secondary metal pipes and a higher power will result in a higher voltage on the secondary metal pipes to affect safety, that is to say, this heating method is limited by power. This is further evidenced by the following formulas: (1) S=K√P, in which the cross-section S of the iron core is directly proportional to the square root of power P, and K is a constant; (2) S=E/4.44ƒBN, in which the electromotive force E (that may be seen as the supply voltage here), the frequency ƒ and the magnetic induction intensity B are considered as certain values so that the cross-section S of the iron core is inversely proportional to the number of turns N; the higher the power, the higher will be the voltage on each turn. In addition, a number of problems, such as oversize, low power, low thermal efficiency and complicated manufacturing process, may also exist with the heating device that its container is integrated in it and the iron core at the yoke area is placed outside the container and exposed to the atmosphere. As for the problems with other existing heating methods, details are given in this patent description.
The working principle for liquid heating described in Chinese Patent ZL97106984.4 is: the iron core is laminated by silicon steel sheets in the shape of “” and the ferromagnetic steel part, i.e. the steel magnet, is positioned on the upper part of the iron core in the shape of “”. The three-phase magnetic conductor made of above two different materials creates a closed three-phase magnetic loop, which connects the windings on the three legs of the iron core in the shape of “” to a three-phase industrial frequency power source. Therefore, a three-phase alternating magnetic flux that is far stronger than the eddy-current and magnetic hysteresis of the iron core is generated in the steel magnet, which is in turn heated rapidly. Here, the major source of heat comes from the eddy-current so that it is briefly called eddy-current heating. Its structure is: the upper end of the metal shell is welded to the lower end of the above steel magnet in an enclosed mode so that the above-mentioned iron core and windings are encapsulated in this metal shell. The leading wires of the windings are led out from a connector base that is positioned on one side of the metal shell. All vacancies in the metal shell as well as in the connector base are packed with insulating fillers so as to form a completely enclosed solid body. When it is used, all its parts but the opening of outgoing lines are immersed in liquid. During operation, the heat generated by the steel magnet, iron core and windings is conducted to the surrounding liquid through the metal shell directly or indirectly. It is obvious here that the heat radiation from the metal shell to the areas surrounding the winding of each phase is uneven, so that winding temperature may rise higher at local areas of the windings between two phases UV and two phases VW to affect the service life. In addition, this device has other inadequacies in its oversized section of iron core and low power factor.